If you have your original serial key then you're fine. If you bought OEM then tougher, especially if you have a Dell, HP, etc. In which case, call up M$, tell them your old mobo fried and you want to put your legit XP onto the new machine. They might ask for your old key, and if they're in a good mood they might just give you one anyways.
OH, and tell them you need XP reinstalled because you're gonna be upgrading to Vista. Whether you are or not, they'll be more willing to help you